fix: round-robin dispatch panics with more than 8 providers

[javi11]

Summary

• Replaces the hard-coded [8]int cumulative-weights array in doSendWithRetry with a dynamically-sized make([]int, n) slice
• Removes the now-unnecessary [:n] slice expression on the sort.SearchInts call
• Adds TestClient_RoundRobinMoreThan8Providers to prevent regression

Root cause

// before
var cumWeights [8]int  // panics when len(mains) > 8
cumWeights[i] = totalW // index out of range [8] with length 8

Any deployment with 9+ main providers hit an immediate runtime error: index out of range panic.

Test plan

• go test -run TestClient_RoundRobinMoreThan8Providers -race ./... passes
• Full test suite passes